Sec. 1-2] 



MECHANICAL INPUT TRANSDUCERS 



31 



R+AJ? 



The resistance is low. When closed, resistances of different trans- 

 ducers vary between 0.1 and 5 ohms; when open, the resistance 

 values range between 3 and 20 ohms. Resistance changes between 

 open and closed position as large as 15 ohms have been observed. 



The error for short-time operation is in the order of 1 to 2 per cent; 

 over small ranges of operation and with selected gauges better values 

 can be obtained. The deviation from linearity of the output charac- 

 teristics is + 2 per cent over limited ranges of the order of 0.001 to 

 0.01 in. Considerable extension of the linear range may be obtained 

 by the use of pairs of matched gauges in a resis- 

 tance bridge. The dynamic response varies widely 

 with the spring construction. Natural frequencies 

 of 250 cps have been observed. The (short-tirne) 

 hysteresis can be considerable; it may vary from 

 less than 1 to 10 per cent. 



Spring transducers may be made from noble 

 metals which are hard enough to have the elastic 

 characteristics of a spring (platinum-rhodium, 

 or "Paliney" alloy, primarily a gold-silver alloy) 

 or from non-noble metals such as steel which, 

 for better contact, should be gold-plated. The 

 wire diameter of practical transducers is between 

 0.005 and 0.015 in. 



The spring transducer is, in principle, an elegant solution of the 

 transducer problem. It is simple and sensitive, its force require- 

 ments are small compared to the wire strain gauge, the impedance is 

 low, and it has the added advantage of requiring only a minimal 

 amount of accessory equipment; but — like all transducer systems 

 based on contact variations — it shows a considerable degree of un- 

 reliability, which limits its application. 



c. Wire Strain Gauges. This strain gauge, Fig. (1-2)13, consists 

 essentially of a fine wire, usually of about 0.001 in. diameter. When 

 exposed to strain within the elastic limit, two physical mechanisms 

 will cause a change of its resistance. First, its geometrical form will 

 vary, i.e., the wire will show an increase of length and a decrease of 

 cross-sectional area. Second, a change of electrical resistivity will 

 occur, which can result in an increase or decrease of resistance, 

 depending upon the material of the wire. 



The strain coefficient of resistance is 



Fig. (1-2)13. Sche- 

 matic diagram of a 

 resistance-wire strain 

 gauge. 



dR , , dp , 



— de = (1 +2,0 +-?de 

 K p 



(1) 



